System for higher-order dispersion compensation including phase modulation

ABSTRACT

A higher-order dispersion compensator including a chromatic dispersion compensator coupled to receive an input signal; a phase modulator optically coupled to the chromatic dispersion compensator, wherein the phase modulator selectively chirps portions of the data pulses; and a tunable dynamic dispersion element coupled to receive the phase modulated signal. The tunable dynamic dispersion element includes a first waveguide having a first non-linearly chirped grating tuned to reflect the polarization controlled signal and having a first reference reflection point; and a first tuning mechanism that tunes the first grating.

RELATED APPLICATIONS

The present application is related to and claims priority from the,commonly assigned U.S. applications entitled, “System for PolarizationMode Dispersion Compensation”, USPTO Ser. No. 10/036,987, now U.S. Pat.6,748,126, and “Method for Polarization Mode Dispersion Compensation”,USPTO Ser. No. 10/037,024, now U.S. Pat. 6,907,199, both of which areincorporated by reference. This application also is related to andclaims priority from provisional application “Method And System ForHigher Order Dispersion Compensation”, USPTO Ser. No. 60/344,965, filedon Dec. 31, 2001, which also is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and system for dispersioncompensation of optical signals. In particular, the present inventionrelates to a method for higher-order dispersion compensation using atleast two chirped Bragg gratings to selectively tune the reflectionpoints of two polarization resolved signals, creating a variablehigher-order dependent delay.

Present day telecommunication systems require that optical signals beconveyed over very long distances. In an optical communications signal,data is sent in a series of optical pulses. Signal pulses are composedof a distribution of optical wavelengths and polarizations, each ofwhich travels at its own characteristic velocity. This variation invelocity leads to pulse spreading and thus signal degradation.Degradation due to the wavelength dependence of the velocity is known aschromatic dispersion, while degradation due to the polarizationdependence is known as polarization mode dispersion (PMD).

Mathematically, the velocity of light v in a waveguide is given by$\begin{matrix}{v = \frac{c}{n}} & (1)\end{matrix}$where c is the velocity of light in free space and n is the effectiveindex of refraction in the waveguide.

Normally, the effective index, n, of the optical medium is dependentupon the wavelength of the light component. Thus, components of lighthaving different wavelengths will travel at different speeds.

In addition to being dependent upon wavelength, the effective index in awaveguide also may be dependent upon the polarization of the opticalsignal. Even in “single-mode” fiber, two orthogonal polarizations aresupported and, in the presence of birefringence, the polarizationstravel at different speeds. Birefringence in the fiber may arise from avariety of sources including both manufacturing variations andtime-dependent environmental factors. The speed difference results in apolarization-dependent travel time or “differential group delay” (DGD)between the two different polarization modes within the birefringentfiber. In real optical fiber systems, the magnitude of birefringence andthe orientation of the birefringent axes vary from place to place alongthe fiber. This results in a more complex effect on the optical signal,which is characterized by the concept of “principal states ofpolarization” or PSPs. PSPs are defined as the two polarization statesthat experience the maximum relative DGD, and they uniquely characterizethe instantaneous state of the system.

Polarization mode dispersion (PMD) is measured as the distortion arisingfrom the statistical sum of the different group velocities of the twocomponents of polarization as the signal propagates through thedifferent sections of the optical communications system. PMD includesfirst order PMD and higher-order PMD and is non-deterministic. Firstorder PMD is the differential polarization group delay at a givenwavelength. The instantaneous value for a long fiber may vary over bothlong time intervals, due to slow variations such as temperature drift,and short time intervals, due to fast variations such as mechanicalvibration induced polarization fluctuations. The coefficient describingthe mean value of first order PMD may vary from more than 2 ps/km/^(1/2)for relatively poor PMD performance fiber to less than 0.1 ps/km^(1/2)for relatively good PMD performance fiber.

Second order PMD arises mainly from two sources: i.) a first order PMDthat varies with wavelength; ii.) a change of the system PSP (principalstate of polarization) orientation with wavelength, which results in avariation of PMD with wavelength. Second order PMD results in awavelength dependent group delay, which is equivalent in effect tovariable chromatic dispersion, and can have either a negative orpositive sign. The speed of fluctuation is on the same order as thespeed of fluctuation of first order PMD.

There are two types of chromatic dispersion: deterministic and variable.Deterministic dispersion is the set chromatic dispersion per unit lengthof waveguide having a fixed index of refraction. Deterministicdispersion is relatively fixed (e.g., ˜17 ps/nm*km for standard singlemode fiber) for a given set of environmental conditions. For example, 17ps/nm*km means that a ten kilometer (10 km) system, carrying data with abandwidth of 0.1 nanometers (nm), will experience approximately 17picoseconds (ps) of chromatic dispersion.

Variable chromatic dispersion is caused by changes in fiber link length,due to adding or dropping channels for example, and by tensile stressesand/or fluctuations in temperature. Reasonable values to be expected forthe amount that the chromatic dispersion will change are in the range−500 ps/nm to +500 ps/nm.

In addition to the effects of PMD and chromatic dispersion alone, thereis a higher-order dispersion cross term that arises from thesimultaneous presence of both chromatic dispersion and PMD. This crossterm between chromatic dispersion and second order PMD has a mean valueof zero, but may have a non-zero root-mean-square (RMS) contribution.Similarly to second order PMD terms, the RMS value may have a positiveor negative contribution. The magnitude of the RMS contribution may varyfrom less than 1% of the chromatic dispersion to the same order as thechromatic dispersion, depending on the PMD coefficient of the fiber.

Dispersion imposes serious limitations on transmission bandwidth,especially across long distances, such as in transoceanic routes.Dispersion issues become much more important at higher bit rates, wherethe separation between the optical pulses is less and where shorterpulses result in a wider signal spectral bandwidth, exacerbatingchromatic and higher-order PMD effects. At bit rates greater than orequal to 40 Gb/s, even for “good” fiber (≲0.1 ps/km^(1/2) PMDcoefficient), long length links are deemed to require higher-orderdynamic compensation. Dispersion may become an inhibiting factor eitherlimiting overall system length or increasing system costs due to theneed for additional optical-to-electrical-to-optical signal conversionsites to permit electrical signal regeneration.

Higher-order dispersion has not been adequately recognized, measured andaddressed in past dispersion compensation devices. An understanding ofthe sources and factors in higher-order dispersion is important inproviding a higher-order dispersion compensation solution.

Exemplary calculations for a “good” fiber (PMD coefficient of 0.1ps/km^(1/2)) show:

Chromatic Dispersion term: 17 ps/nm*km First order PMD coefficient: 0.1ps/km^(1/2) Second order PMD coefficient: 0.006 ps/nm*km Cross term RMSmagnitude: 0.37 ps/nm*kmExemplary calculations for a “poor” fiber (1 ps/km^(1/2)) show:

Chromatic Dispersion term: 17 ps/nm*km First order PMD coefficient: 1ps/km^(1/2) Second order PMD coefficient: 0.6 ps/nm*km Cross term RMSmagnitude: 3.7 ps/nm*km

The second order coefficient of PMD may be calculated based on thetheory described in “Second-Order Polarization Mode Dispersion: Impacton Analog and Digital Transmissions,” IEEE J. of Lightwave Tech.,JLT-16, No. 5, pp. 757–771, May 1998, which is hereby incorporated byreference.Second order PMD coefficient=(First order PMD coefficient)²/1.73  (2)

Equation 2 only accounts for the root-mean-square (RMS) of the resultingchromatic dispersion. The cross term was calculated to be:Cross term=(17)^(1/2)*(First order PMD coefficient)^(1/2)*1.16  (3)

Therefore, it may be appreciated that for fiber that has a high PMDcoefficient, PMD may cause a problem when only fixed chromaticdispersion compensation is used due to accumulated chromatic dispersionthrough the second order PMD term and the cross term. This leads to ahigh value of uncompensated dispersion as fiber PMD coefficients becomelarger or as the bit rate gets higher.

From this analysis, it may be calculated that even using the best offiber produced today (assuming ˜0.025 ps/km^(1/2)), propagationdistances are likely limited to ≲3000 km (dispersion<0.3*100 ps) for 10Gb/s transmission and ≲200 km (dispersion<0.3*25 ps) for 40 Gb/s withoutperforming dynamic chromatic dispersion compensation to eliminate theeffects of the 2nd order PMD and cross terms.

A number of literature articles attempt to address the issue ofhigher-order dispersion compensation. One approach is to use amulti-section PMD compensator. Such an approach is likely to beexpensive and also will be limited in the amount of variable chromaticdispersion compensation available. Another approach is to selectivelyadd specific chirps to various portions of the pulse and to send thepulse through a high dispersion element with the correct sign tocompress the pulse. Such an approach may account for all types ofdispersion. However, such an approach is likely to be expensive due tothe need for clock recovery and phase modulation and also only may beuseable at the receiver terminal. Furthermore, it only may work if theresidual dispersion is low.

The need remains for a dispersion compensation system that dynamicallyadjusts not only for PMD, but also for chromatic dispersion andhigher-order dispersion. Increased telecommunications systemrequirements, such as the need to compensate for fluctuations intemperature and the possibility of variable path lengths due to theoptical add/drop systems envisioned in the near future, call for acompensation system that is dynamic and cost-efficient.

SUMMARY OF THE INVENTION

The present invention relates to a higher-order dispersion compensatorfor tuning a signal having a first polarization mode dispersioncomponent, a second order polarization mode dispersion component, and avariable chromatic dispersion component.

The compensator includes a first tuning element that adjusts the firstorder polarization mode dispersion component of the polarizationcontrolled signal and a second tuning element that adjusts the secondorder polarization mode dispersion component and the variable chromaticdispersion component of the polarization controlled signal. Thecompensator may further include a polarization controller that convertsincoming light of an arbitrary polarization to a polarization controlledsignal having a desired state of polarization.

In one embodiment, the first tuning element may comprise a differentialhigher-order delay line including a polarization beam splitter/combinercoupled to receive the polarization controlled signal, where thepolarization beam splitter splits the polarization controlled signalinto a first polarization component and a second orthogonal polarizationcomponent. A first waveguide having a first grating tuned to reflect thefirst polarization component and a first reference reflection point isoptically coupled to receive the first polarization signal. A secondwaveguide having a second grating tuned to reflect the secondpolarization component and having a second reference reflection point isoptically coupled to receive the second polarization signal. The firstwaveguide has a first chirped grating tuned to reflect the firstpolarization signal at a first reference reflection point. The secondwaveguide is optically coupled and aligned to the second output port ofthe splitter. The second waveguide has a second chirped grating tuned toreflect the second polarization signal and has a second referencereflection point. Depending on the embodiment of the present invention,the chirp of the gratings may be linear, non-linear, or may have a morecomplex spatial dependence. For certain applications, the length of thegratings may be equal or greater than one meter.

In this first exemplary embodiment, both gratings are Bragg gratingslinearly chirped to perform first order PMD compensation and fixedchromatic dispersion compensation. A first tuning mechanism tunes one ofthe gratings, such as by mechanically stressing the gratings.

In other embodiments, both gratings may be non-linearly chirped toperform first and second order PMD compensation as well as both fixedand variable chromatic dispersion compensation. Other, more complicated,chirp patterns may be chosen to perform more specific or higher-order(third, fourth, etc. order) compensation.

The first grating and the second grating may both have substantiallysame reflection profiles and substantially same chirp rates; and thefirst and second reference reflection points may be at substantially asame optical path length. Alternatively, prior to adjustment by thetuning mechanism, one of the reflection points of the gratings may be ata shorter optical path length from the split point than the secondreflection point.

The second tuning element may include a third waveguide having a thirdnon-linearly chirped Bragg grating and a second tuning mechanism thattunes the third grating. The range of chirp values in the thirdnon-linearly chirped Bragg grating may determine the relative range ofvariable chromatic dispersion compensation.

The system may further include a static chromatic dispersion componentcomprising an average chirp rate of the first and second gratings thatcorresponds to the amount of fixed chromatic dispersion to becompensated.

Circulators may be used to route the optical signals. One embodimentincludes a four-port circulator, the circulator having an input portoptically coupled to receive the polarization control signal, a firstrecirculation port optically coupled to transmit the controller outputsignal to the differential polarization delay line and to receive thedelay line output, a second recirculation port optically coupled totransmit the delay line output signal to the second tuning element andto a second tuning element output signal, and an output port opticallycoupled to transmit a final output signal.

In another exemplary embodiment, the first tuning element and the secondtuning element comprise a polarization beam splitter coupled to receivethe polarization controlled signal, where the polarization beam splittersplits the polarization controlled signal into a first polarizationcomponent and a second orthogonal polarization component. A firstwaveguide is optically coupled to receive the first polarization signal,the first waveguide having a first non-linearly chirped grating tuned toreflect the first polarization signal and having a first referencereflection point. A second waveguide is optically coupled to receive thesecond polarization signal, the second waveguide having a secondnon-linearly chirped grating tuned to reflect the second polarizationsignal and having a second reference reflection point. A first tuningmechanism tunes both the first and the second grating simultaneously anda second tuning mechanism tunes the second grating independently of thefirst grating. The compensator may have a static chromatic dispersioncompensation element, wherein the average chirp rate of the first andsecond gratings correspond to the amount of fixed chromatic dispersionto be compensated.

In this embodiment, the first order polarization mode dispersioncompensation element includes the second tuning mechanism and the secondgrating and the first order polarization mode dispersion compensation isachieved by tuning the second non-linearly chirped grating separatelyfrom the first grating. The second order polarization mode dispersioncompensation and variable chromatic dispersion compensation elementsinclude the first and second grating and the first tuning mechanism andvariable chromatic dispersion compensation and higher-order polarizationmode dispersion compensation are achieved by tuning the first and secondgratings in unison.

In yet another embodiment, the higher-order dispersion compensatorcomprises a chromatic dispersion compensator coupled to receive an inputsignal; a phase modulator optically coupled to the chromatic dispersioncompensator, wherein the phase modulator selectively chirps portions ofthe data pulses; and a tunable dynamic dispersion element coupled toreceive the phase modulated signal. The tunable dynamic dispersionelement includes a first waveguide having a first non-linearly chirpedgrating tuned to reflect the polarization controlled signal and having afirst reference reflection point; and a first tuning mechanism thattunes the first grating.

The compensator may include a signal analyzer optically coupled toevaluate the signal reflected by the grating and provide control signalsto the tuning mechanism accordingly. The signal analyzer further mayprovide control signals to the phase modulator.

The waveguides are exemplarily optical fibers. In specific embodiments,the waveguides may be optical single-mode polarization-maintaining (PM)fibers, polarizing (PZ) fibers, and/or shaped optical fibers.

The compensator may be an adaptive compensator further including asignal analyzer, which provides control signals to at least one of thetuning mechanisms. The dispersion compensator may be at least partiallyintegrated into an integrated optical chip, such as a lithium niobatechip. The waveguides may be channel waveguides. Alternative tuningmechanisms may tune the gratings acoustically, thermally,electro-optically, or mechanically.

A method for compensating for higher-order dispersion of an incomingoptical communications signal in accordance with the present inventionincludes the steps of compensating the signal for first orderpolarization mode dispersion; compensating the signal for second orderpolarization mode dispersion; and compensating the signal for variablechromatic dispersion. Additionally, the method may include the steps ofcompensation for fixed chromatic dispersion and controlling thepolarization of the incoming signal. The signal may be monitored afterthe compensating steps and the degree of compensation may be tuned basedon the monitoring.

The step of compensating the signal for first order polarization modedispersion may include the steps of controlling the polarization of thesignal; splitting the signal into a first and a second orthogonalpolarization components; reflecting the first polarization component ina fixed linearly chirped grating; reflecting the second polarizationcomponent in a tuned linearly chirped grating; and recombining the firstand the second polarization components.

The step of compensating the signal for second order polarization modedispersion may comprise the step of reflecting the signal in a tunednon-linearly chirped grating.

In a particular embodiment of the method of the present invention, themethod includes the steps of:

-   -   adjusting the state of polarization of the incoming optical        communications signal to correctly align the principal states of        polarization of the communications signal to the principal        states of polarization of the compensator system;    -   splitting the communications signal into a first and a second        orthogonal principal states of polarizations at a split point;    -   directing the first of the polarization states to a first        waveguide having a first non-linearly chirped grating having a        first reference reflection point;    -   directing the second of the polarization states to a second        waveguide having a second non-linearly chirped grating having a        chirp pattern substantially similar to that of the first chirped        grating and having a second reference reflection point;    -   adjustably varying the chromatic dispersion of the first and        second reflections by varying the position of the first and        second reflection points along the gratings;    -   adjustably varying the relative optical path lengths between the        first and second reflection points and the split point to        compensate for polarization dispersion between the first and        second orthogonal states of polarization; and    -   recombining the first and second polarization states into an        output signal.

The method may further include the steps of sampling the quality of theoutput signal. Using the quality readings, the method may include thesteps of:

-   -   adaptively adjusting the state of polarization of the incoming        signal and the optical path length of the second reflection        point with respect to the split point to compensate for        first-order polarization mode dispersion in response to the        quality of the output signal, and/or    -   adaptively adjusting one or both of the first and second        reflection points with respect to the split point in order to        compensate for the dispersion in the signals.

Prior to the step of adjustably varying the optical path length from thesecond reflection point, the optical path length of at least one of thegratings may be tuned such that the second reflection point is at adesired point, for example, at substantially the same optical pathlength or at a different path length from the split point as the firstreflection point. The difference may be selected according to anexpected polarization dispersion delay between the first and secondorthogonal states of polarization.

In another embodiment of a method for compensating for higher-orderdispersion of an optical communications signal in accordance with thepresent invention, the method comprises the steps of:

-   -   splitting the communications signal into a first and a second        orthogonal principal polarization states;    -   directing the first polarization state to a first        high-birefringence optical waveguide having a first linearly        chirped grating, the first optical waveguide having a first        reflection point at a first optical path length;    -   directing the second polarization states to a second tunable        high-birefringence optical waveguide having a second linearly        chirped grating, the second optical waveguide having a second        reflection point at a second optical path length;    -   recombining the first and second polarization states into an        output signal;    -   directing the output signal to a third high-birefringence        optical waveguide having a non-linearly chirped grating with a        reflection point;    -   adjustably varying the second optical path length of the second        linearly chirped grating to compensate for polarization        dispersion between the first and second orthogonal states of        polarization; and    -   adjustably varying the optical path in the third grating to        compensate for higher-order dispersion in the output signal.

The second chirped grating may have a chirp pattern substantiallysimilar to the first chirped grating, the second grating having a secondreflection point that is substantially at the same optical path lengthfrom the split point as the first reflection point. Again, the outputsignal ay be sampled the state of polarization of the incoming signaland the optical path length of the second reflection point may beadjusted in response to the quality of the output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic component flow diagram for the dynamic,higher-order dispersion compensation method in accordance with thepresent invention.

FIG. 2 is a schematic representation of a first embodiment of a dynamic,higher-order dispersion compensation system in accordance with thepresent invention.

FIG. 3 is a schematic representation of a second embodiment of a dynamichigher-order dispersion compensator in accordance with the presentinvention.

FIG. 4 is a schematic representation of a third embodiment of a dynamic,higher-order dispersion compensator in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method and a system forhigher-order dispersion compensation (HDC), which may simultaneouslycompensate for the effects of higher-order dispersion composed ofchromatic dispersion, polarization mode dispersion, higher-orderpolarization mode dispersion, and variable chromatic dispersion whileminimizing optical loss and equipment overhead. Higher-order dispersioncompensation is defined to include chromatic, polarization mode, secondorder polarization mode, and variable chromatic dispersion compensation.

An exemplary embodiment of the present invention relates to an adaptivehigher-order dispersion compensator system. The system includes apolarization controller, a differential polarization delay unit, adynamic chromatic dispersion compensation element, and a fixed chromaticdispersion element. In various forms of the present invention, thesetasks are performed by two, three, or four elements. For example, in oneembodiment all four tasks may be performed by separate elements, whereasin another embodiment all tasks except polarization control areperformed by one compensation element.

Chirped reflection gratings in waveguides, such as fiber Bragg gratings(FBGs), are used to create a time delay between orthogonalpolarizations. In one exemplary embodiment of the current invention, atleast two chirped Bragg gratings are used to create a time delay betweenorthogonal polarizations, while inducing a correcting amount ofchromatic dispersion, depending on the chirp of the gratings and thetuning level of the gratings. A signal analysis method indicates thelevel of a specific dispersion component, or the level of all dispersioncomponents, and a control scheme will use the signal analysis results tooptimize the system.

Signals to be compensated may include a first polarization modedispersion component, a second (and/or higher) order polarization modedispersion component, a variable (or dynamic) chromatic dispersioncomponent, and/or a static (or fixed) chromatic dispersion component.FIG. 1 is a process diagram of the steps of an exemplary method used inproviding higher-order dispersion compensation to a signal according tothe present invention. The incoming signal 142 traverses a polarizationcontroller 140, which converts light of an arbitrary incomingpolarization to a controlled output signal 144 having a desired state ofpolarization. The controlled output signal goes through a fixedchromatic dispersion compensator 112 that minimizes the total pulsedistortion. The amount of negative chromatic dispersion introduceddepends on the expected link length leading up to the compensator. Formany of the embodiments discussed, this function will be performedsimultaneously with the following step due to the use of chirped fiberBragg gratings.

The signal output is sent through a first order PMD compensator 114.Then, the resulting signal is sent through a variable chromaticdispersion compensator 116, which compensates both for any dynamicchromatic dispersion as well as any residual second order PMD. Finally,the resulting compensated output signal 148 is monitored through anoptical signal tap 118 by a signal analysis module 120, which outputscontrol signals 128 to control components, such as the polarizationcontroller 140 and the appropriate dispersion compensation elements 110.

The grating period Λ that will most strongly reflect an optical signalof a vacuum wavelength λ is given byΛ=λ/2n  (4)where n is the effective index of the waveguide. Thus, by controllingΛ(x), the reflection point and therefore the propagation time, of thesignal pulse in the grating may be controlled. Furthermore, thechromatic dispersion imparted to a signal being reflected from a chirpedgrating of period Λ(x) which has a corresponding gradient in period (or“chirp”) dΛ/dx along the grating is given byD=(c*dΛ/dx)⁻¹  (5)

By properly designing and tuning the grating period Λ(x), one maycontrol both the chromatic dispersion and the total delay experienced bythe pulse in the grating.

The type and number of dispersion measurement techniques used may varydepending on the situation. FIG. 1 illustrates the use of threetechniques: DOP Measurement 122, Sub-harmonic filtering 124, and/orfrequency modulation 126. There may be advantages to using two or moremeasurement techniques simultaneously, such as degree of polarizationmonitoring and electrical sub-harmonic frequency filtering. However,this may add to the cost of the compensator and therefore it may bedesirable to use only one technique. Other measurement techniques orvariations of the techniques shown may be advantageous. Some examplesare: transversal filtering; DOP measurement with scanning filter; ordirect eye analysis using a Bit Error Rate Tester or DigitalCommunications Analyzer. Both feed-forward and feedback measurementtechniques may be used appropriately to analyze the signal.

Certain compensation elements from the embodiment illustrated in FIG. 1could be combined with various feed-forward approaches or with a phasemodulation compensation approach to get additional functionality.

FIG. 2 is a schematic diagram of a first embodiment of a higher ordercompensation system 200 in accordance with the current invention wherethe second order dispersion and the first order dispersion arecompensated for by separate compensation elements. A communicationssignal 242 enters the system through polarization controller 240,wherein the polarization state of the incoming signal 242 is modified.

The resulting controller output signal 244 then goes through a firstcirculator 250. The circulator 250 has an input port coupled to receivethe controller output signal 244, a recirculation port optically coupledto transmit the controller output signal to a first order PMD and fixedchromatic dispersion compensation element comprising, in the presentexample, a differential delay line 260, (and to provide a return pathfor the reflected signal), and an output port coupled to transmit thereflected signal 246 to another circulator 252.

The differential delay line 260 includes a polarization beamsplitter/combiner 262, a first fixed delay element 270, and a secondtunable delay element 280. The polarization beam splitter 262 splits thecontroller output signal into two orthogonal polarization components.One polarization component travels down the first fixed delay element,which exemplarily includes a first aligned waveguide 272 having a firstlinearly chirped Bragg grating 274. The second polarization componenttravels down the second delay element 280, exemplarily comprising asecond waveguide 282 with a second linearly chirped Bragg grating 284.Other, more complicated, chirp patterns may be chosen to perform morespecific or higher-order (third, fourth, etc. order) compensation. Thewaveguides may be birefringent, thereby suppressing coupling between thetwo polarization modes in each. The differential delay element includesa first waveguide and a second waveguide. The waveguides and the outputports of the splitter are optically coupled and aligned by matchingtheir cores and polarization axes.

The signals are reflected at a reflection point inside the gratings 274and 284 corresponding to the wavelength of the signal. This point may bevaried in the second waveguide 282 by tuning the grating 284 usingtuning mechanism 286.

The tuning mechanism 286 is capable of variably adjusting the opticalpath length of one or both of the reference points, with respect to thesplit point, by changing the effective period of the grating elements tochange the reference reflection point of the signal. Mechanisms fortuning the grating period may include: applying axial mechanical stressto stretch or compress the gratings, applying electric fields toelectro-optically control the grating index, applying heat tothermo-optically control the grating index, or using other tuningmechanisms known in the art, such as mechanisms that tune the gratingsacoustically and/or mechanically (e.g., by stretching or compressing thegratings).

The waveguides 272 and 282 are exemplarily optical fibers. In specificembodiments, the waveguides 272 and 282 may be optical single-modepolarization-maintaining (PM) fibers, polarizing (PZ) fibers, and/orshaped optical fibers, such as those described in commonly-owned,co-pending U.S. application Ser. No. 09/515,187, and U.S. Pat. No.6,459,838, which are both hereby incorporated by reference.

When the signal returns to circulator 250 upon reflection, it will nowbe compensated for both first order PMD, because of the optical pathlength difference between the two gratings' reflection points, and fixedchromatic dispersion, because the gratings have a predetermined chirprate that corresponds to the amount of fixed chromatic dispersion to becompensated. Thus, the average chirp rate of the first grating 274 andthe second grating 284 determines the amount of fixed chromaticdispersion compensation.

In the present exemplary embodiment, the first grating and the secondgrating both have substantially the same reflection profiles andsubstantially the same chirp rates; and the first and second referencereflection points are at substantially the same optical path length.

Alternatively, prior to adjustment by the first tuning mechanism, thefirst reflection point of the first grating may be at a shorter or at alonger optical path length from the split point of the beam splitterthan the second reflection point. The initial position of the first andsecond reference reflection points with respect to the split point(i.e., the optical path length of the segment) may be tailored to theparticular application. In applications where the expected DGD does notexceed the range of the tuning mechanism, the first and second referencereflection points may be at substantially the same optical path lengthwith respect to the split point. Alternatively, one or the otherreference reflection points may be biased, that is, may have a differentoptical path length, to compensate for all or part of the first orderPMD.

Different components of the present invention may be integrated into anintegrated optical device, such as a LiNbO₃ chip, which containsbirefringent waveguides. In one embodiment, the polarization controllerand the differential delay line are integrated onto a single LiNbO₃chip. In another embodiment, the polarization dispersion compensatorcomponents from neighboring channels in a wavelength divisionmultiplexing (WDM) system may be integrated onto a single LiNbO₃ chip.Obviously, integrated optical devices based on other materials systemscould also be used.

The signal 246 then goes through a second circulator 252, which providesa return path for the signal upon reflection from a variable chromatic,higher order PMD dispersion compensator 264. The compensator 264includes a third delay element 290 including a waveguide 292, whichcontains a tuned third grating 294. In this instance, the grating 294 isnon-linearly chirped.

Exemplarily, the waveguides 272, 282 and 292 are single-mode opticalfibers. In an exemplary embodiment, the fibers are polarizationmaintaining fibers. By tuning the grating 294 appropriately using tuningmechanism 296 (such as by applying a stress or temperature gradient),both variable chromatic and higher-order polarization mode dispersionwill be dynamically compensated. The amount of chirp in the linearlychirped gratings 274 and 284 must be adjusted to take into account theaverage chromatic dispersion induced by the non-linearly chirped grating294 in the succeeding section. The range of chirp values in thenon-linearly chirped waveguide grating will then determine the relativerange of variable chromatic dispersion compensation.

The illustrated three grating configuration allows for a very flexiblerange of PMD, chromatic, and variable chromatic dispersion compensationamounts without changing the layout, due to the flexibility in writingdifferent chirp and bandwidth gratings.

Referring back to FIG. 2, an optical tap coupler may be coupled to theoutput port of the circulator to provide a sample of the output signalto the signal analyzer. The analyzer evaluates the quality of the delayline output signal and provides control signals to the polarizationcontroller and the differential polarization delay unit. Control signals228 go to the polarization controller 240, the tuning mechanism for thelinearly chirped waveguide grating 284 and the tuned non-linearlychirped waveguide grating 294 from a signal analysis module 220 thatsamples output signal 248. As mentioned previously, the preferreddetection methods may depend on the situation.

The arrangement of the dispersion compensation elements in FIG. 2 isexemplary. For example, one may place the variablechromatic/higher-order PMD compensator 264 before the first orderPMD/fixed chromatic dispersion compensator 260. One also may use asecond polarization controller (not illustrated) before the variablechromatic/higher-order PMD compensator 264 to get higher functionalitydue to added first order PMD compensation if the tuned non-linearlychirped waveguide grating is a fiber Bragg grating (FBG) written inpolarization-maintaining fiber (PMF). Also, the two three-portcirculators 250 and 252 could be replaced with a single four-portcirculator, eliminating one component.

Another compensator 300, which can accomplish dynamic higher-orderdispersion compensation, is shown in the FIG. 3. In the compensator 300,the signal 342 first passes through a fixed chromatic dispersioncompensator 312, such that only first order PMD and higher orderdispersion components (second order PMD and variable chromaticdispersion) remain. It then goes through a first and second order PMDand dynamic chromatic dispersion compensator 390 comprising a phasemodulator 380 and a waveguide 392 containing a non-linearly chirpedBragg grating 394. The phase modulator 380 adds chirp selectively intime, to parts of the signal. A phase correction module 382 provides acontrol signal 327 for the phase modulator. The purpose of the phasecorrection module 382 is to align the phase and period of the chirpingafforded by the modulator with the appropriate phase and period of theincoming signal, for example an NRZ encoded signal. The chirped signalthen goes through a circulator 350 and into the waveguide 392 containingthe non-linearly chirped Bragg grating 394. The non-linearly chirpedBragg grating 394 is tuned by a tuning mechanism 396 to have the properlevel of dispersion. The combination of the chirp applied to parts ofthe signal and the dispersion imparted by the tuned grating 394 willhave the effect of compressing the edges of the pulse in time, therebycompensating for any residual and/or higher order dispersion components.

After returning from the Bragg grating 394, the signal 348 leaves thecirculator 350 through an output recirculation port, and is sampled by asignal analysis module 320. The signal analysis module 320 supplies theappropriate control signals 328 to the tuning mechanism 396 toadaptively tune the non-linearly chirped Bragg grating 394. An exemplarycandidate for the signal analysis is sub-harmonic filtering since thephase modulator and dispersive element (non-linearly chirped FBG)combination compensates for all types of dispersion. Also, an exemplarylocation for this method of compensation is at the receiver end of alink, where an electrical representation of the signal is present to tapoff from. In this case, there would be a receiver after the circulator350, such that the signal 348 would be electrical rather than optical.

This approach may be used in combination with other elements. Forexample, a first order PMD compensator may be used in conjunction withcompensator 300 to reduce the polarization dispersion to be compensated.This would be highly advantageous in links with high PMD to reduce thenecessary compensation range of the dynamic higher-order dispersioncompensator.

An alternative embodiment of FIG. 3 would have the signal analysismodule 320 control both the tuned grating 392 and the phase correctionmodule 380 to optimize the combined performance of the chirp added tothe signal by the modulator and the dispersion experienced in thegrating.

Another exemplary embodiment would use no fixed chromatic dispersioncompensation element 312 or a substantially lower or more convenientvalue of fixed dispersion for the fixed chromatic dispersioncompensation element 312. In this case, the relative range of thetunable dispersion element 394 could be adjusted to the proper level tocompensate for all dispersion terms. That is, the tunable dispersionelement 394 could be adjusted to compensate for fixed and variablechromatic dispersion and the phase modulator 380 could be adjusted suchthat the combination of the added phase and the dispersion element 394could compensate for the remaining dispersion. This may afford theoverall compensation system much more flexibility and range during thecompensation process.

Another exemplary embodiment would have the non-linearly chirped grating394 written into a highly birefringent waveguide, for examplepolarization maintaining fiber, and further include a polarizationcontroller to control the polarization of the signal entering thenon-linearly chirped grating 394. This embodiment could be implementedeither of the two ways described previously—with or without a fixedchromatic dispersion compensation element 312. In the case of the fixedchromatic dispersion compensation element 312 being present thecombination of polarization control and a non-linearly chirped Bragggrating could be used to compensate for all or part of the first orderpolarization mode dispersion. The fixed chromatic dispersion element 312would compensate for the fixed chromatic dispersion present in theincoming signal. Then, the combination of the phase modulation and theinstantaneous value of dispersion could be optimized to compensate forthe remaining dispersion components. In the case of the fixed chromaticdispersion compensation element 312 not being present, the remainingterms to be compensated for by the combination of the phase modulationand the dispersion element would also include the fixed chromaticdispersion term.

FIG. 4 illustrates another embodiment of a compensation system 400 inaccordance with the current invention. The system 400 allows forintegration of the higher-order dispersion compensation concept intofewer stages. The input signal 442 enters the system 400 through apolarization controller 440, which converts the incoming polarizationstate of the signal into a signal 444 with a desired state ofpolarization. After the signal 444 goes through the recirculation portof a circulator 450, it is optically coupled to a polarization beamsplitter/combiner 462 of a delay assembly 460. The delay assembly alsoincludes a first delay element 470 and a second delay element 480. Thesignal 444 is split into its orthogonal components, and each componentis directed to an output port of the splitter/combiner 462. The signalssimultaneously traverse the first delay element 470, which includes afirst waveguide 472 containing a first non-linearly chirped grating 474and a second waveguide 482 containing a second non-linearly chirpedgrating 484, and the second delay element 480, which includes a secondwaveguide 482 containing a second non-linearly chirped grating 484.

The first grating 474 and the second grating 484 have a tuning mechanism490 that controls both gratings simultaneously. The second grating 484has an additional tuning mechanism 492 that tunes the grating 484independently. Fixed chromatic dispersion compensation is accomplishedby writing the correct average value of chirp into the non-linearlychirped waveguide gratings 474 and 484. Variable chromatic dispersioncompensation and higher-order PMD compensation are achieved by tuningthe gratings 474 and 484 in unison, by changing both their temperaturesby the same amount, for example. First order PMD compensation isachieved by tuning the second non-linearly chirped grating 484separately, by stressing it for example. The types of tuning that may beused are not limited to temperature or stress tuning, and either may beused for the unison or separate tuning.

Upon reflection from the gratings, the compensated signal 446 isrecombined by the polarization splitter/combiner 462, and proceeds backthrough the circulator 450 to the circulator output port. An exemplaryembodiment of the current invention would include an optical tap coupler452 and a signal analysis module 420 after the circulator 450. Dependingon the detection methods chosen and whether or not feed-forward orfeedback algorithms are used, the tap 452 may be located in a differentpart of the system. The signal analysis module 420 provides controlsignals 428 to the tuning mechanisms 490 and 492 and to the polarizationcontroller 440.

The method and systems described above may result in a small amount ofresidual fixed chromatic dispersion induced between the two polarizationcomponents of the signal. For small first order PMD amounts this islikely to be acceptable, as shown in the calculation below of anexemplary system based on fiber Bragg gratings.

Fixed Chromatic Dispersion Amount: 700 ps/nm Variable ChromaticDispersion Amount: −500 ps/nm to +500 ps/nm Actual Grating Chirp Ranges:200 ps/nm to 1200 ps/nm (or 50 pm/mm to 8.33 pm/mm period chirp) GratingLength (example): 1 meter Chirp rate change per length: 10 ps/nm per cmDGD Range (example): 100 ps (or 1 cm reflection point change) GratingChirp rate change 10 ps/nm per DGD max Range:

In an exemplary system, if the incoming first order PMD (DGD) were themaximum expected 100 ps, a grating of the above design may add aresidual chromatic dispersion of 10 ps/nm preferentially between the twoorthogonal polarization components of the signal. This amount wouldusually be much lower, since PMD has a Maxwellian distribution withtime, such that if the maximum expected amount were 100 ps, the actualamount seen most of the time would be much less. Furthermore, increasingthe grating length or changing the design otherwise could reduce theamount further. The above configuration will lead to an extremelyflexible, low loss method of compensating for any type or anycombinations of types of dispersion in a compact form factor.

In the exemplary embodiment of FIG. 4, the waveguides 472 and 482 aremade of polarization maintaining fiber. In an alternate embodiment, thewaveguides may be integrated on an integrated optical chip, such as alithium niobate chip. Further, it may be advantageous to incorporatemore of the components of the compensator system into the integratedoptical chip, such as the polarization splitter/combiner 462 and thepolarization controller 440. An alternative layout for FIG. 4 would beto position the polarization controller 440 between the circulator 450and the polarization splitter/combiner 462. This would facilitate theintegration of some or all of these components together onto an opticalchip. For example, an exemplary embodiment may integrate thepolarization controller 440, the polarization splitter/combiner 462, andthe waveguides 472 and 482 onto one substrate.

Those skilled in the art will appreciate that the present invention maybe used in a variety of optical applications where higher-ordercompensation is desired. While the present invention has been describedwith a reference to exemplary preferred embodiments, the invention maybe embodied in other specific forms without departing from the spirit ofthe invention. Accordingly, it should be understood that the embodimentsdescribed and illustrated herein are only exemplary and should not beconsidered as limiting the scope of the present invention. Othervariations and modifications may be made in accordance with the spiritand scope of the present invention.

1. An adaptive higher-order dispersion compensator comprising: a) achromatic dispersion compensator coupled to receive an input signal; b)a phase modulator optically coupled to the chromatic dispersioncompensator, wherein the phase modulator selectively chirps portions ofdata pulses; and c) a tunable dynamic dispersion element coupled toreceive a phase modulated signal, wherein the tunable dynamic dispersionelement comprises i) a first waveguide having a first non-linearlychirped grating tuned to reflect the polarization controlled signal andhaving a first reference reflection point; and ii) a first tuningmechanism that tunes the first grating.
 2. The compensator of claim 1,further comprising a signal analyzer optically coupled to evaluate thesignal reflected by the grating and provide control signals to thetuning mechanism accordingly.
 3. The compensator of claim 1, wherein thesignal analyzer further provides control signals to the phase modulator.4. The compensator of claim 1, further comprising a circulator whichreceives the signal from the phase modulator, directs the signal to thefirst waveguide, receives the signal reflected by the grating andredirects it to an output port.
 5. The compensator of claim 1, whereinthe first waveguide is an optical fiber.
 6. The compensator of claim 1,wherein the first waveguide is an optical single-modepolarization-maintaining fiber.
 7. The compensator of claim 1, whereinthe waveguide is an optical single-mode polarizing fiber.
 8. Thecompensator of claim 1, wherein the waveguide is a shaped optical fiber.9. The compensator of claim 1, where the grating is a fiber Bragggratings.
 10. The compensator of claim 1, where the dispersioncompensator is at least partially integrated into an integrated opticalchip.
 11. The dispersion compensator of claim 10, where the integratedoptical chip is a lithium niobate chip.
 12. The compensator of claim 1,wherein the waveguide is a channel waveguide in an integrated opticalchip.
 13. The compensator of claim 1, wherein the first grating measuresat least one meter in length.
 14. An adaptive higher-order dispersioncompensator comprising: a) a phase modulator coupled to receive a signalincluding data pulses, wherein the phase modulator selectively chirpsportions of the data pulses, and b) a tunable dynamic dispersion elementcoupled to receive the signal, wherein the tunable dynamic dispersionelement comprises i) a first waveguide having a first non-linearlychirped grating; and ii) a first tuning mechanism that tunes the firstgrating.
 15. The compensator of claim 14, wherein the tunable dynamicdispersion element further comprises: a polarization controller coupledto receive output from the phase modulator, wherein the polarizationcontroller controls the state of polarization of the signal and whereinthe first waveguide comprises a first highly-birefringent waveguide. 16.The compensator of claim 14, further comprising a signal analyzeroptically coupled to evaluate the signal and provide control signals tothe tuning mechanism accordingly to compensate for instantaneouschromatic dispersion.
 17. The compensator of claim 16, wherein thesignal analyzer further provides control signals to the phase modulatorto compensate for remaining components of dispersion in the incomingsignal.
 18. The compensator of claim 14, further comprising a circulatorwhich receives the signal from the phase modulator, directs the signalto the first waveguide, receives the signal reflected by the grating andredirects it to an output port.
 19. The dispersion compensator of claim14, wherein the waveguide is an optical fiber.
 20. The dispersioncompensator of claim 14, wherein the waveguide is an optical single-modepolarization-maintaining fiber.
 21. The dispersion compensator of claim14, wherein the waveguide is an optical single-mode polarizing fiber.22. The dispersion compensator of claim 14, wherein the waveguide is ashaped optical fiber.
 23. The dispersion compensator of claim 14, wherethe grating is a fiber Bragg grating.
 24. The dispersion compensator ofclaim 14, where the dispersion compensator is at least partiallyintegrated into an integrated optical chip.
 25. The dispersioncompensator of claim 24, where the integrated optical chip is a lithiumniobate chip.
 26. The dispersion compensator of claim 14, wherein thewaveguide is a channel waveguide in an integrated optical chip.
 27. Thedispersion compensator of claim 14, wherein the first grating measuresat least one meter in length.
 28. An adaptive higher-order dispersioncompensator comprising: a) a chromatic dispersion compensator coupled toreceive an input signal having data pulses; b) a phase modulator coupledto receive output from the chromatic dispersion compensator, wherein thephase modulator selectively chirps portions of the data pulses; c) apolarization controller coupled to receive output from the phasemodulator, wherein the polarization controller controls the state ofpolarization of the signal to a desired state; d) a tunable dynamicdispersion element coupled to receive output from the polarizationcontroller, the dispersion element comprising: i) a first highlybirefringent waveguide having a first non-linearly chirped grating tunedto reflect the polarization controlled signal and having a firstreference reflection point; and ii) a first tuning mechanism that tunesthe first grating.
 29. The compensator of claim 28, wherein the firstwaveguide is a polarization-maintaining optical fiber.
 30. Thecompensator of claim 28, further comprising a signal analyzer opticallycoupled to evaluate the signal reflected by the grating and providecontrol signals to the tuning mechanism accordingly.
 31. The compensatorof claim 30, wherein the signal analyzer further provides controlsignals to the phase modulator.
 32. The compensator of claim 30, whereinthe signal analyzer further provides control signals to the polarizationcontroller.
 33. The compensator of claim 28, further comprising acirculator which receives the signal from the phase modulator, directsthe signal to the polarization controller, receives the signal reflectedby the grating and redirects it to an output port.
 34. The dispersioncompensator of claim 28, wherein the waveguide is a shaped opticalfiber.
 35. The dispersion compensator of claim 28, where the grating isa fiber Bragg grating.
 36. The dispersion compensator of claim 28, wherethe dispersion compensator is at least partially integrated into anintegrated optical chip.
 37. The dispersion compensator of claim 36,where the integrated optical chip is a lithium niobate chip.
 38. Thedispersion compensator of claim 28, wherein the waveguide is a channelwaveguide in an integrated optical chip.
 39. The dispersion compensatorof claim 28, wherein the first grating measures at least one meter inlength.
 40. An adaptive higher-order dispersion compensator comprising:a) a phase modulator coupled to receive a signal including data pulses,wherein the phase modulator selectively chirps portions of the datapulses; b) a tunable dynamic dispersion element coupled to receive thesignal; and c) a signal analyzer optically coupled to evaluate thesignal and provide control signals to the tunable dynamic dispersionelement accordingly to compensate for instantaneous chromaticdispersion.